Pressure swing adsorption (PSA) is commonly used to separate a multicomponent feed gas into two streams: a primary product at high pressure, rich in one or more mildly adsorbed components and a low pressure secondary product, rich in one or more strongly adsorbed components. Any intermediate components that are moderately adsorbed are split between the two products.
The operating sequence for a conventional 2-bed PSA process typically consists of bed pressurization, product release, and regeneration stages. In the prior art it is common to also include in the operating sequence, a bed pressure equalization step, in order to reduce the pressure from which the bed to be regenerated is vented.
For a given bed design, sieve capacity and operating pressure, the time periods for each of the steps of the operating sequence mentioned above are normally chosen so that when the concentration of the strongly adsorbed component reaches a specified limit in the primary product, the product release step is halted and the bed is regenerated. For a system operating under these conditions, a major loss of weakly adsorbed components occurs due to their presence in the gas located in the bed void space or weakly bound to the adsorbent which is vented during the regeneration stage. The higher the operating pressure, the greater is this loss.
U.S. Pat. No. 4,340,398, issued to Doshi et al. discloses a process for a PSA system containing three or more beds wherein void gas is transferred to a tank prior to bed regeneration, thereby reducing the pressure from which the bed vents, and later using this gas for repressurization. The primary purpose of Doshi et al.'s invention is to separate the product backfill step and bed equalization step, in order to obtain a more uniform product with no discontinuity. The separation of the two steps is achieved through a storage vessel which primarily serves as a buffer tank for backfill gas. Doshi et al. suggest equalization of the bed to the tank simultaneously with the product backfill into the tank. Consequently, only a small portion of the void gas can be drawn into the tank and, hence, the advantage of tank equalization in the present invention is not fully realized by Doshi et al.
Void gas loss can be minimized by reducing the pressure from which the bed vents. A process modification to the 2-bed PSA, incorporating tank equalization has been proposed in the literature. (See, e.g. Lee, H. and Stahl, D. E., "Pressure Equalization and Purging System for Heatless Adsorption Systems", U.S. Pat. No. 3,788,036, issued on Jan. 29, 1974; Marsh, W. D., Holce, R. C., Pramuk, F. S. and Skarstrom, C. W., "Pressure Equalization depressuring in heatless adsorption", U.S. Pat. No. 3,142,547, issued on July 28, 1964). The modification proposed in both patents, which has been recommended for zeolite PSA systems producing either oxygen from air or hydrogen from a hydrogen/hydrocarbon mixture, consists of equalizing the bed with a tank after bed equalization in order to transfer part of the bed void gas, and subsequently using the conserved gas as purge gas to aid regeneration of the bed. The tank gas, obtained from the bed void space, contains less of the desired (mildly adsorbed) component than the product gas. Consequently, for a process using product purge for regeneration, the tank modification reduces the product purge requirement and improves the yield. In the single bed PSA embodiment suggested in U.S. Pat. No. 3,788,036, two tanks are used; the first equalization tank is used to conserve void gas for repressurization while the void gas conserved by a second equalization tank is used as a purge. The first equalization tank in this case fulfills the function of the bed pressure equalization in the 2-bed process. The process suggested with 2-beds includes only one tank which conserves void gas for use as purge.
Void gas conserved by pressure equalization with a tank can be used either as a purge gas or for repressurization. In order to improve the yield of the mildly adsorbed components in the primary products by repressurization, conserved void gas for repressurization can only contain strongly adsorbed components in concentrations slightly above that in the primary product itself. If the strongly absorbed component is present in concentrations much higher than the product specification, the conserved void gas can at best be used as purge gas and the improved yield of the weakly adsorbed component is modest. Even when used as a purge gas, the conserved void gas can only contain strongly adsorbed components in concentrations lower than their corresponding feed concentration. At the end of the production cycle, the void gas is very rich in the mildly absorbed components and the concentration of strongly adsorbed components is very small and close to that in the primary product. Hence it is important to save this gas for repressurization. The bed pressure equalization in a 2-bed process is included for this purpose. At the end of bed pressure equalization, the pressure in the bed that has just been produced is lower and the concentration of the strongly adsorbed component starts to build up in the bed void gas due to desorption as the equilibrium changes with lower pressure. Therefore, when only stongly or mildly adsorbed components are present, tank equalization following bed pressure equalization to conserve void gas can only be used to provide purge gas for bed regeneration. Mildly adsorbed components in the void gas are lost in the purge gas.
The above analysis applies to systems which contain mildly adsorbed or strongly adsorbed components. This is the case in the separations described in U.S. Pat. No. 3,788,036 (oxygen from nitrogen) and in U.S. Pat. No. 3,142,547 (hydrogen from hydrocarbons). The present invention is particularly concerned with separations involving multicomponent gas mixtures which contain moderately adsorbed component(s) apart from strongly adsorbed and mildly adsorbd components. Downstream processing requires that the primary product contain less than a specified concentration of the strongly adsorbed components. The moderately adsorbed component(s) can, however, be tolerated either in the primary product or in the secondary product over a range of concentrations. The separation of multicomponent systems that come under this category are slightly different from systems studied hitherto and hence permit effective use of one or more tank equalizations following bed pressure equalization for the purpose of conserving void gas for subsequent bed repressurization.
In the case of systems containing a moderately adsorbed component, the void gas (at the end of the production cycle) is rich in mildly adsorbed components and contain small to moderate amounts of moderately adsorbed components and low concentrations, at or below product specification, of strongly adsorbed components. Following bed pressure equalization, the moderately adsorbed components readily desorb and strongly adsorbed components are still in the adsorbed phase. The void gas is now rich in the moderately adsorbed component with significant amounts of the mildly adsorbed component. This gas is conserved by tank equalization and still meets the requirement for use as repressurization gas to improve yield. The tank equalizations can be continued until the strongly adsorbed components start to desorb and their concentration level in the void gas becomes appreciable. The moderately adsorbed component(s) thus provide a buffer range over which tank equalizations are effective for producing repressurization gas. The yield of the mildly adsorbed component is improved appreciably through the tank equalizations with a concurrent change in the ratio in which the moderately adsorbed component(s) distributes in the primary and secondary products.
The combination of a 2-bed PSA with one or more pressure equalization tanks for the purpose of improving yield of mildly adsorbed components in the primary product by conserving gas for subsequent use in bed repressurization is unique to this invention. This combination is very effective when one or more of the following conditions are met:
(i) A moderately adsorbed component that is permitted to distribute between the primary and secondary products is present.
(ii) The feed gas is at very high pressure and the PSA operation is carried out at pressures higher than the optimum PSA operative pressure for the purpose of conserving pressure energy in the primary product for downstream processing. In this case, the tank equalization provides a mechanism by which the desorbing pressure can be maintained at optimum level irrespective of the adsorption pressure, thereby reducing the loss of mildly adsorbed (desired) component during venting.
(iii) Desired product is very valuable and hence purge containing even small amounts of this gas is not recommended. Vacuum regeneration will then be favored in spite of high associated costs.
(iv) Purge gas is available from an external source and so the conserved void gas can be used for repressurization in its entirety.
An object of the present invention is to increase yield and reduce void gas loss in a 2-bed PSA system separating a multicomponent as feed. Another object of the present invention is to reduce the pressure from which the beds in a PSA system are vented. A further object of the present invention is to employ a single tank or a plurality of tanks to conserve the void gas in a PSA system by equalization with each bed before and after regeneration. Yet a further object of the present invention is to use the tank equalizations as a mechanism to enable PSA operation at high pressures to conserve available pressure energy in the feed for downstream processing while carrying out desorption from very low pressures.